pfstools: unpin OpenCV 2

This builds fine with OpenCV 4 using `enableUnfree`.

pkgs/tools/graphics/pfstools/default.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, mkDerivation, fetchurl, cmake, pkg-config, darwin
 , openexr, zlib, imagemagick6, libGLU, libGL, libglut, fftwFloat
 , fftw, gsl, libexif, perl, qtbase, netpbm
-, enableUnfree ? false, opencv2
+, enableUnfree ? false, opencv
 }:
 
 mkDerivation rec {
@@ -36,7 +36,7 @@ mkDerivation rec {
     OpenGL GLUT
   ]) else [
     libGLU libGL libglut
-  ]) ++ lib.optional enableUnfree (opencv2.override { enableUnfree = true; });
+  ]) ++ lib.optional enableUnfree (opencv.override { enableUnfree = true; });
 
   patches = [ ./glut.patch ./threads.patch ./pfstools.patch ./pfsalign.patch ];